1. Field of the Invention
The invention relates to the field of cleaning semiconductor wafers and more particularly to the field of cleaning silicon semiconductor wafers.
2. Prior Art
Freshly sawn, lapped or ground silicon wafers are extremely dirty and must be cleaned, if subsequent electronic device fabrication processes are to be successful. Among the components of the dirt on the wafers are spindle oil; handcream; silicon particles; silicon powder; cooling solution, including wetting agents; lapping and polishing grit; epoxy casting compounds; human fingerprints and possibly other materials. If this dirt is not removed from the wafers, subsequent processing steps are adversely affected.
In attempting to find an adequate cleaning solution for cleaning all types of wafers, the assignee's materials preparation department evaluated more than 50 commercially available compositions. Most wafers can be cleaned in acid baths. However, heavily doped P-type wafers have required 20% hot KOH and even then do not come out really clean. Further, the hot KOH is reported to leach the dopant from the heavily doped P-type wafers. No single commercial cleaning composition was found which would adequately clean all wafers types and doping levels. Even the best materials for a given wafer type often left wafers dirty.
As a means of determining wafer cleanliness, the assignee's material preparation department utilized a swab test comprising dipping a cotton swab in methylene chloride and scrubbing it across the wafer. The wafer was only considered clean if the swab looked clean following the scrubbing of the wafer. Many wafers cleaned in the best prior art cleaners required recleaning because they failed the swab test after their initial cleaning. Recleaning increases production costs and reduces yields because of increased wafer breakage.
The best of the acidic commercial solutions was selected for use in cleaning all wafers except the heavily doped P-type wafers. This solution cleans approximately 4,000 wafers in 18 gallons of cleaning solution before the solution must be discarded. The wafers are immersed in this solution for seven seconds.
An improved wafer cleaning technique is needed which assures the production of clean wafers in an economical manner. A single cleaning system for all wafers is highly desirable, as is one which does not leach the dopant from the heavily doped P-type wafers.